Jason W. Barnes

We analyze a specular reflection of the Sun off of Titan’s largest sea, Kraken Mare, observed by the Cassini Visual and Infrared Mapping Spectrometer on the T104 flyby (2014 August 21). We use the specularly reflected I / F signal to derive a transmission spectrum. Owing to the low incidence and emission angles (51°) of the reflection, the specular signal is visible down to the 1.3 and 1.6 μ m windows, as well as the 2.0, 2.7/2.8, and 5 μ m windows as had been seen previously on T85. We confirm the T85 result that Titan’s atmosphere absorbs more at 2.7 μ m than it does at 2.8 μ m and that the “notch” between those windows results from absorption in the atmosphere and not from the surface. We compare our derived T104 spectrum to numerically integrated optical depths calculated using Huygens-derived haze properties and correlated- k gas absorption coefficients. We fit the T104 observation using 63% ± 3% greater haze abundance than derived from Huygens and a wavelength exponent of 2.50 ± 0.03, steeper than the 2.0 derived from Huygens. We measure a surface methane fraction of 0.043 ± 0.001 over Kraken Mare at 68°N latitude, slightly lower than the Huygens measurement of 0.0539 ± 0.0014 in Titan’s tropics. The comparison shows the utility of specular reflection transmission spectra for quantitative determination of Titan’s atmospheric properties. Fitting of carbon monoxide (CO) absorption places a constraint of 22.6 ± 0.3 ppm on the fraction of CO in Titan’s atmosphere. Mismatches between our model and these specular transmission observations may allow quantitative estimation of the effects of unknown composition gaseous absorbers throughout Titan’s atmosphere.

Titan, with its thick and hazy atmosphere, is a key world in our solar system for understanding light scattering processes. NASA’s Cassini mission monitored Titan between 2004 and 2017, where the derived data set includes a large number of whole disk observations. Once spatially integrated, these whole disk observations reveal Titan’s phase-dependent brightness, which can serve as an analog for how hazy worlds might appear around other stars. To explore Titan’s phase curve, we present a pipeline for whole disk Titan observations acquired by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) spanning 0.9–5.1 μm. Application of the pipeline finds over 4400 quality spatially and spectrally resolved data cubes that were then integrated over Titan’s disk to yield phase curves spanning 2°–165° in phase angle. Spectra at near-full phase provide a useful approximation for Titan’s geometric albedo, thus extending the spectral coverage of previous work. Crescent phase brightness enhancements in the Cassini/VIMS phase curves are often more extreme than analogous results seen at optical wavelengths, which can be explained by atmospheric transparency and haze scattering processes. These results provide validation opportunities for exoplanet-focused spectral models and also shed light on how extreme aerosol forward scattering could influence exoplanet observations and interpretations.

Background/Motivation:Titan’s atmosphere is an organic factory, producing high-order hydrocarbons from the photolysis of methane by solar ultraviolet radiation [Lavvas et al. 2008]. These organics aggregate together to form tholins, which then sediment out of the atmosphere to the surface. Constraining the temporal evolution and distribution of Titan’s aerosols is required to understand the roles of haze production in the Titan system including radiative heating, dynamics, atmospheric chemistry, and surface erosion/dune formation. Studies of Titan’s hazes have demonstrated significant variability in haze abundance over the course of the Cassini mission and beyond [Rannou et al. 2010, West et al. 2018, Nicols-Fleming et al. 2021]. We seek to expand on this work by considering newly analyzed data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) in combination with state-of-the-art radiative transfer (RT) models.Here, we present an analysis of all the VIMS solar occultation data over the course of the Cassini mission. These unique data offer the ability to sample fine vertical structure in both gaseous composition and aerosol abundance. Past studies have leveraged the occultation data occurring in the first half of the mission [Bellucci et al. 2009, Hayne et al. 2014, Maltagliati et al. 2015, Rannou et al. 2022]. Now, we double the dataset, including data from the second half of the mission, providing both a longer temporal baseline and greater spatial coverage through which to understand the formation and distribution of Titan’s aerosols. The final profile of derived vertical abundance permits both a study of the spatio-temporal variability in Titan’s aerosols and more accurate atmospheric profiles for RT modelling, which generally assume constant profiles as derived from the Huygens probe. Accurate RT is critical for atmospheric compensation and interpretation of retrieved surface properties.Methods:Occultation observations were attempted on 10 different flybys of Titan over the Cassini mission, with 6 flybys offering data on both the ingress and egress of the flyby for a total of 16 datasets. However, of these 16 datasets, one resulted in no data of the Sun, which was like result of bad VIMS pointing while an additional 7 datasets suffered from significant variability because of Cassini’s thrusters being activated during the acquisition resulting in significant drifts in intensity as the Sun moved about the focal plane. As the occultation requires an accurate baseline of the solar flux prior to the event to determine relative transmission, these observations were discarded from the analysis.Of the remaining 8 datasets, all were analyzed with a similar approach built around the methodology as described in [Maltagliati et al. 2015]. First, a 2D Guassian was fit to each wavelength-dependent image from the occultation and removed to determine the background residuals. For cases where stray light from Titan was present through the non-solar port, a 2D plane was used to fit and remove the background residuals. After, the 2D Gaussian was refit and the total flux summed across the focal plane. Transmission was then determined for each timestep through division by the average solar flux taken from all altitudes greater than 1000 km. As a final correction, a linear polynomial was fit to all transmission data to account for any additional slow variability in source drift over the course of the acquisition.Preliminary Results:Figure 1 displays the results of the occultation data reduction. As evident in Figure 1, significant variability in sampling resolution is observed between the datasets. This results from differences in the integration time, image frame size, and distance of Cassini from Titan at the time of acquisition. Resolutions range from 5-40 km on average. All plots have been color coded to correspond to similar altitudes ranging from 50-500 km. Strong absorptions are observed around 2.3- and 3.3-µm from methane, with additional weaker absorptions around 1.2- and 1.4-µm. Also evident in most observations is the strong carbon monoxide (CO) doublet at 4.8-µm. However, this signature appears to be significantly weaker in later observations, suggesting the potential for spatio-temporal variability in CO on Titan. Variability in the spectral slopes in the continuum from 1.0 - 2.2-µm also demonstrate changes in vertical aerosol abundance over the course of the observations. Most notable is decreased transmission on the T103/T116 ingress datasets, corresponding to similar latitudes at ~30°N, at ~150km altitudes. These data suggest a significant increase in aerosol abundance at mid-latitudes later in the Cassini mission and will be used to place constraints on seasonal circulation of Titan’s atmosphere.Figure 1: Derived spectral transmission files from eight occultation datasets range from the T10 flyby (Jan. 2006) to the T116 flyby (Feb. 2016). The color corresponds to the sampled altitude for each profile. Strong absorptions from CH4 and CO are observable in the spectra. Variability in spectral slopes are indicative at changes to the vertical aerosol abundance between flybys.

NASA’s fourth New Frontiers mission, Dragonfly (Turtle & Lorenz 2024), will land a robotic octocopter on Saturn’s moon Titan in 2034 to study its prebiotic chemistry, constrain its habitability, and search for potential chemical biosignatures. The target landing site will be a portion of the Shangri-La sand sea just south of Selk Crater, an 80-km impact structure (Lorenz et al. 2021). Taking advantage of Titan’s low gravity and thick air, Dragonfly will aerially traverse to over twenty distinct landing sites on dune, interdune, and icy crater terrains (Lorenz et al. 2018).Figure 1. Dragonfly is a large 4.5-meter long, almost 1000 kg aerial rover with a sophisticated and high-mass scientific payload. It will spend its 3.3-year nominal surface mission exploring and sampling Titan’s sand dunes, interdunes, and the icy 80-km Selk Crater impact structure.On the ground at those sites we will employ our four scientific instruments: a mass spectrometer (DraMS Trainer et al. 2022), a gamma-ray/neutron spectrometer (DraGNS), seven cameras (DragonCam), and a geophysical and meteorological suite (DraGMet). The DrACO sampling system will drill into the surface and ingest surface samples by means of a hydraulic vacuum cleaner. The vehicle spends over 99% of its time on the ground, and therefore while it can and will fly, the term “relocatable lander” brings to mind a better sense for the operations that we expect to execute on Titan.We seek to determine how far organic chemistry has progressed, to ground-truth the global methane meteorological cycle, to measure the modes and rates of surface geologic processes, to constrain when and where water and organics might have mixed, and to look for evidence that either water- or hydrocarbon-based life may have existed on Titan (Barnes et al. 2021). Our science objectives with respect to organic chemistry are to measure compositions of materials in different geologic settings and to determine presence and abundance of key molecules for Earth-like life. We have goals to characterize Titan’s methane cycle by constraining the atmospheric methane moisture budget at our tropical landing site and assaying the abundance of stored liquid methane in the near-subsurface.To place our samples into context, we will need to assess their provenance which we will do by determining the conditions for aeolian transport, by determining the transport mode and history of clastic saterials, and by establishing the geologic context of sampled materials. Our fourth goal is to determine where and how liquid water may have mixed with organic material, which we will do with a seismometer to measure current lithospheric activity and, if Titan is sufficiently active seismically, to constrain the depth to Titan’s liquid-water ocean. Other instrumentation will determine the availability of near-surface water ice and constrain past geologic processes.Dragonfly finally has a goal to search for any potential chemical biosignatures on Titan as may have been produced by either extinct or extant life. Our objectives on this front are to determine enantiomeric abundance of chiral molecules, to determine if patterns exist in molecular masses and distribution, and to follow up on Huygens hydrogen indications with more and more accurate hydrogen profiles of the lower atmosphere.Dragonfly recently passed its Critical Design Review and we are fabricating hardware ahead of the launch period that opens on 2028 July 5.Figure 2. Dragonfly is a single-element mission, and will communicate to Earth directly by means of an 85-cm radial line slot array high-gain antenna, as can be seen here deployed on the ventral (top) side of the vehicle. REFERENCESBarnes, J. W., Turtle, E. P., Trainer, M. G., et al. 2021,Science Goals and Objectives for the Dragonfly TitanRotorcraft Relocatable Lander, The Planetary ScienceJournal, 2, 130, doi: 10.3847/PSJ/abfdcfLorenz, R. D., Turtle, E. P., Barnes, J. W., et al. 2018,Dragonfly: a Rotorcraft Lander Concept for scientificexploration at Titan, Johns Hopkins APL TechnicalDigest, 374Lorenz, R. D., MacKenzie, S. M., Neish, C. D., et al. 2021,Selection and Characteristics of the Dragonfly LandingSite near Selk Crater, Titan, The Planetary ScienceJournal, 2, 24, doi: 10.3847/PSJ/abd08fTrainer, M. G., Brinckerhoff, W. B., Grubisic, A., et al.2022, Dragonfly Mass Spectrometer Investigation atTitan, in The Astrobiology Science Conference(AbSciCon) 2022, 239–03Turtle, E. P., & Lorenz, R. D. 2024, Dragonfly: In SituAerial Exploration to Understand Titan’s PrebioticChemistry and Habitability, in 2024 IEEE AerospaceConference, IEEE, 1–5

We present new evidence for active coastal and oceanic features in Titan’s Punga Mare observed in a high-phase Cassini Visual Infrared and Mapping Spectrometer observation of sun glint from the T110 flyby. We observe sunglint in a coastal channel, Apanohuaya Flumen, resulting from differing pixel contributions of land adjacent to the liquid-filled channel. Along the eastern shoreline, we identify a 5 μm bright margin. A possible explanation for this brightening includes a coastal margin of capillary wave fields. We find additional evidence of variegated sea surface roughness in Fundy Sinus and isolated Sun glitter near Hawaiki Insulae. RADAR observations of debouches (where rivers meet bays) within Punga Mare overlap several bright 5 μm pixels that indicate rough liquid surfaces. We postulate that a change in liquid flow regimes, possibly occurring as surface streamflow or bubble outburst events, may be responsible for surface roughness near these debouches. These observations imply air–sea–land interactions and active hydrology represented by possible streamflow are present in Titan’s sea district during the northern summer.

The surface of Titan is composed of varied geomorphic units indicative of a vivid depositional, erosional and tectonic history. Dominant at Titan's equatorial regions are vast eolian landscapes of dunes, sand sheets, yardangs and wind streaks. These features reveal the action of wind in time and space, which has moved and shaped unconsolidated materials across the surface. Cassini Visual and Infrared Mapping Spectrometer (VIMS) spectra of these materials are most consistent with organics, and particle sizes from Cassini Synthetic Aperture Radar (SAR) vary from dust to cobbles [1]. Most grains appear to be sand-sized, based on the predominance of SAR-dark (smooth at 2 cm) material organized into dune forms at the equator, and models for wind requirements to build dunes on Titan [2]. Dunes cover ~15% of Titan’s surface, or 13 million km2 [3, 4] in the form of linear dunes 1-2 km wide and spaced by 1-4 km [5]. This type and size of dune is dominant in the Arabian and African deserts and is also known as longitudinal [6], referring to the net direction of the transport of sand, which aligns with dune crests [7, 8]. All dunes visible in the Cassini SAR imagery have now been traced down their long axes, revealing regional and global orientations (proxy for wind) and distributions, controlled by obstacles and regional elevations. Over 30,000 dunes have been traced (Fig. 1), and given SAR image coverage, this represents ~40% of all dunes thought to be present based on sand sea locations from the Cassini Imaging Science Subsystem (ISS), meaning we may expect as many as 75,000 dunes to exist on Titan. A total dune length of 1.309 million km has now been calculated from the measured dunes; extrapolated to all of Titan, this length would be 3.273 million km. Using an average dune width of 1 km and corresponding height of 100 m from a 0.1-0.2 dune height:width relationship [9] and isolated measurements [10] we find a total measured dune sand volume of 130,900 km3. If scaled up to all dunes thought to be present, the total volume of sand from dunes alone of 327,250 km3 falls within previous estimates for sand volumes [11, 4, 12]; however, those studies also included sand sheets without detectable dunes. Thus, this new measurement may reveal greater sand volumes from dunes alone than previously thought. This represents a significant material volume that has been moved through wind action, even if from locations close to the (unknown) source. At higher latitudes, several distinct regions of SAR-bright dunes or wind-carved ridges or yardangs are also present (Fig. 2). These can be found in flat plains [13], where they may be inactive dunes, or on the elevated, fluvially carved domes of the midlatitudes, postulated to be volcanic laccoliths [14]. Ash or otherwise derived fine organic grains would be ideally soft yardang materials, comparable to those on Earth, and reveal that winds can also remove substrate on Titan, similar to the action of wind in locations on Mars [15]. Statistical comparisons between possible yardangs support their existence on the domes but are consistent with them being dunes in other locations [16]. The dome yardangs would have resulted from removal of fine-grained materials and loss to the atmosphere as airborne particles and eventually other surfaces – new estimates of the volume of yardang erosion on one small dome, based on yardang heights of ~100 m from shadows, is ~125 km3, and there are a handful of identified other similar yardang regions. Creation of dust from dune sand movement has not yet been estimated (and there are significant uncertainties about material properties) but with yardang processes could yield large volumes that now reside in unknown locations, including interdunes, high latitude plains, lakes and seas. SAR and ISS bright streaks are visible at low to midlatitudes on Titan, similar in orientation to the dunes where they overlap, and found behind obstacles [17, 18]. Their SAR brightness is consistent with a larger particle size, which may be supported by lower density materials, but wind streaks are also often identified with fine particle sizes. This reveals yet another sand and dust location on Titan. Yardangs and wind streaks, along with the ubiquitous equatorial dunes, confirm a general W-E flow of wind, with some N or S deviations that may be reflections of a global undulatory wind pattern at high latitudes. The Dragonfly mission will study dune regions in depth and will reveal particle sizes and compositions, nature of the interdune, level of dune activity and sand (and perhaps dust) movement at the present day, and magnitude and direction of modern winds [19]. These observations will anchor our understanding of eolian processes in the Solar System. [1] Le Gall, A. et al. 2010, Icarus 207, 948-958. [2] Lorenz, R.D. et al. 2006, Science 312, 724-727. [3] Le Gall, A. et al. 2011, Icarus 213, 608-624. [4] Rodriguez, S. et al. 2014, Icarus 230, 168-179. [5] Radebaugh, J. et al. 2008, Icarus 194, 690-703. [6] Courrech du Pont et al. 2024; Earth-Science Reviews, 104772. [7] Lorenz and Radebaugh 2009; Geophysical Research Letters 36. [8] Lucas et al. 2014; Geophysical Research Letters 41, 6093-6100. [9] Lancaster, N. 1995, Geomorphology of Desert Dunes. [10] Neish, C.D. et al. 2010. Icarus 208, 385-394. [11] Arnold, K. 2013, BYU MS Thesis. [12] Lorenz, R.D. et al. 2008, Geophysical Research Letters 35. [13] Lopes, R. et al. 2020. Nature Astronomy 4, 228-233. [14] Schurmeier, L. et al. 2023, Icarus 404, 115664. [15] Kerber, L. et al. 2011. Icarus 216, 212-220. [16] Northrup, D. et al. in progress. [17] Malaska, M.J. et al. 2016. Icarus 270, 183-196. [18] Cohen-Zada, A. et al. 2016. Aeolian Research 20, 108-125. [19] Barnes, J.W. et al. 2021. The Planetary Science Journal 2, 130.Fig. 1. Dunes traced in Shangri-La region of Titan. ISS basemap. Dunes may still be present in large, dark areas, though there is not SAR coverage.Fig. 2. SAR-bright...

We document the evolution of the north–south asymmetry (NSA) of Titan’s haze albedo during the Cassini mission between 2004 and 2017. We analyze coadded cube images taken at 96 distinct wavelengths between 0.35 and 1.05 μm by the Cassini Visual and Infrared Mapping Spectrometer (VIMS-V) instrument from 14 Titan flybys. Over half of a Titan year, we observe a near-complete transition in the NSA boundary latitude across the geographic equator from the southern to the northern hemisphere, including a 3 yr fading of the boundary for several years after the equinox. The fading transition of the NSA matches previous observations of a reversal of the NSA in Hubble Space Telescope images of Titan before the winter solstice between 1997 and 2000. A comparison of NSA images taken at similar times but different phase angles shows the NSA boundary is detectable, albeit with less contrast, at moderately high phase angles (∼90°). Analysis of the NSA boundary in T61 and T67 VIMS images further supports a small tilt between the superrotating atmosphere and the solid body of Titan, as suggested in a previous analysis of 0.890 μm images from the Cassini Imaging Science Subsystem.

With the advent of direct-imaging spectroscopy, the number of spectra from brown dwarfs and extrasolar gas giants is growing rapidly. Many brown dwarfs and extrasolar gas giants exhibit spectroscopic and photometric variability, which is likely the result of weather patterns. However, for the foreseeable future, point-source observations will be the only viable method to extract brown dwarf and exoplanet spectra. Models have been able to reproduce the observed variability, but ground-truth observations are required to verify their results. To that end, we provide visual and near-infrared spectra of Jupiter and Saturn obtained from the Cassini VIMS instrument. We disk-integrate the VIMS spectral cubes to simulate the spectra of Jupiter and Saturn as if they were directly imaged exoplanets or brown dwarfs. We present six empirical disk-integrated spectra for both Jupiter and Saturn with phase coverage of 1.°7–133.°5 and 39.°6–110.°2, respectively. To understand the constituents of these disk-integrated spectra, we also provide end-member (single-feature) spectra for permutations of illumination and cloud density, as well as for Saturn’s rings. In tandem, these disk-integrated and end-member spectra provide the ground truth needed to analyze point-source spectra from extrasolar gas giants and brown dwarfs. Lastly, we discuss the impact that icy rings, such as Saturn’s, have on disk-integrated spectra and consider the feasibility of inferring the presence of rings from direct-imaging spectra.